Known in the art is a process for producing polycrystals of cubic boron nitride comprising application of a high pressure of from 60 to 90 kbar and a temperature of from 1,800.degree. to 2,800.degree. C. to hexagonal boron nitride (cf. French Pat. No. 2,129,200). The final product has good cutting properties in impactless turning, however, it operates unsatisfactorily upon machining of workpieces with a discrete or intermittent surface (i.e. the surface having recesses, grooves, openings), since it possesses insuffivient mechanical strength. A higher strength is inherent in the polycrystals produced by the process taught in U.S. Pat. No. 3,212,852, wherein the treatment of the starting hexagonal boron nitride is conducted under a pressure of above 100 kbar. However, the final product is less suitable for machining of discrete surfaces of workpieces due to a rapid wear thereof. British Pat. No. 1,513,990 teaches a process for producing polycrystals of a superhard material of dense modifications of boron nitride, comprising application of a pressure of from 50 to 90 kbar at a temperature within the range of from 1,800.degree. to 2,800.degree. C. onto hexagonal or wurtzite boron nitride, whereinto additionally introduced is aluminium, boron or aluminium borides uniformly distributed over the entire volume thereof. As to its cutting ability and wear-resistance, the super-hard material produced according to said British Pat. No. 1,513,990 is substantially superior to the above-described materials, though its wear-resistance is considerably reduced upon machining of workpieces with discrete surfaces. The reason for such a low wear-resistance of the material disclosed in British Pat. No. 1,513,990 resides in its low compression strength.
Therefore, improvement of strength characteristics of polycrystals of cubic boron nitride will make it possible to enhance their wear-resistance in machining of parts having intermittent surface, i.e. increase their resistance to shock loads.